Crate flat_enum

About flat_enum crate

This crate expands nesting enums. See the example:

#[derive(FlatTarget)]
pub enum Enum1<A> {
    E1(A),
    E2(),
    E3(String),
}

#[into_flat(Enum2Flat<A>)]
pub enum Enum2<A> {
    #[flatten]
    Enum1(Enum1<A>),
    E4,
}

#[flat(Enum2<A>)]
pub enum Enum2Flat<A> {}

In macro invocation, the Enum2Flat expands into something like:

pub enum Enum2Flat<A> {
    E1(A),
    E2(),
    E3(String),
    E4,
}

In this example, Enum1 and Enum2 are not required to be defined in the same crate. But Enum2 and Enum2Flat should be defined in the same context (module).

Motivation

Memory compaction

In Rust’s enum representation on memory, we have std::mem::Disctiminant value in addition to the field values of each variants. If two enums are nesting, it should have two discriminants on memory. The compiler’s optimization algorithm does not do such work.

This crate gives a way to generate flattened enum automatically to deal with the problem.

Syntax sugar

When using a value of nested enum types in match-like expression, the matchers are easily to become complex. The flattened enum solves that.

Traits

• Flat
  See flat
• FlatTarget
  Marker trait implemented with #[derive(FlatTarget)].
• IntoFlat
  See into_flat

Attribute Macros

• flat
  This attribute macro implements trait Flat on the speficied enum. The definition body should be empty, thus it is generated from corresponding structured enum automatically.
• into_flat
  This attribute macro implements trait IntoFlat on the speficied enum.

Derive Macros

• FlatTarget
  Implements trait FlatTarget on the specified enum. This trait is required to be used as the field type of nesting enum variant augmented with #[flatten] attribute in #[into_flat] enum.